Unravelling Photochemical Relationships Among Natural Products from Aplysia dactylomela

Aplydactone ( 1 ) is a brominated ladderane sesquiterpenoid that was isolated from the sea hare Aplysia dactylomela together with the chamigranes dactylone ( 2 ) and 10- epi -dactylone ( 3 ). Given the habitat of A. dactylomela , it seems likely that 1 is formed from 2 through a photochemical [2 + 2] cycloaddition. Here, we disclose a concise synthesis of 1 , 2 , and 3 that was guided by excited state theory and relied on several highly stereoselective transformations. Our experiments and calculations confirm the photochemical origin of 1 and explain why it is formed as the sole isomer. Irradiation of 3 with long wavelength UV light resulted in a [2 + 2] cycloaddition that proceeded with opposite regioselectivity. On the basis of this finding, it seems likely that the resulting regioisomer, termed "8- epi -isoaplydactone", could also be found in A. dactylomela .